1. Field of the Invention
The present invention relates to wireless communication systems, methods therefor, and base station apparatuses, and more specifically, to a cellular wireless communication system for performing diversity transmission or channel allocation and the like through each base station apparatus having a plurality of antennas, a method thereof, and the base station apparatus.
2. Description of the Related Art
(1) Cellular Communication
Mobile wireless communication generally uses the cellular system to cover a wide service area. In the cellular system, a plurality of base station apparatuses are placed scatteringly in the service area, and the areas covered by the individual base station apparatuses (areas where terminals can communicate) are combined to implement the wide service area. Each base station apparatus sends a reference signal for letting the base station apparatus be recognized. The reference signal is provided as a unique signal in a local area, in terms of transmission signal sequence, transmission time, frequency, or a combination of the transmission sequence, the transmission time, and the frequency. The terminal receives the unique reference signals sent from the base station apparatuses, measures and compares their signal intensities, and recognizes the radio conditions of the local station and adjacent base station apparatuses. The result of radio condition measurement is used to find a base station apparatus providing a higher signal intensity and better reception (and probably the shortest propagation distance). If it is judged that the base station apparatus providing the best reception changes from the currently connected base station apparatus to a different adjacent base station apparatus, a handover process is performed to switch to the base station apparatus that is excepted to provide better reception. In that way, cellular communication is implemented.
(2) Wireless Communication System
FIG. 1 shows the configuration of a wireless communication system.
The concept of cellular communication will be described again with reference to FIG. 1. As shown in the figure, the system has base station apparatuses 20, 21, and 22. A terminal 1 is communicating with the base station apparatus 20 by radio. The base station apparatuses 20, 21, and 22 are connected to a network apparatus 50 to have a wired communication path. An IP connection via a packet switch apparatus 40 is made between the network apparatus 50 and the base station apparatuses 20, 21, and 22. In the figure, the terminal 1 is communicating with the base station apparatus 20, which is the nearest and can provide good signal reception. The base station apparatuses 20, 21, and 22 send their reference signals, which are their identification signals. The terminal 1 receives the reference signals sent by the base station apparatuses and measures their signal intensities. The terminal 1 determines that the base station apparatus that provides the reference signal with the highest intensity is the nearest base station apparatus. The figure shows a downlink signal 30 (from the base station apparatus to the terminal) and an uplink signal 31 (from the terminal to the base station apparatus) of the base station apparatus 20. The base station apparatus 20 sends the downlink signal 30, the base station apparatus 21 sends a downlink signal 32, and the base station apparatus 22 sends a downlink signal 33. Since the downlink signals 30, 32, and 33 are sent at the same frequency at the same time, they can interfere with one another. The terminal 1 is located around the edge of the cell, and therefore receives a desired signal 30 from the base station apparatus 20 and also receives the signals 32 and 33 as interference waves from the other stations at the same time. The terminal 1 is affected by those signals. The ratio of interfering power and noise power to the desired signal power is called a signal interference and noise power ratio (SINR). Around the edge of the cell, interference from an adjacent cell increases and becomes a dominant term of the denominator. This worsens the SINR, making it difficult to transfer information at a high throughput.